Pathogenic calcium crystals, including calcium pyrophosphate (CPPD) and basic calcium phosphate (BCP) crystals are common components of osteoarthritic joints. These crystals identify a subset of patients with unusually severe, rapidly progressive, arthritis. Yet, how crystals form in the normally unmineralized articular cartilage matrix remains unknown. Matrix vesicles are membrane-bound, chondrocyte-derived, extracellular organelles implicated in calcium crystal formation. Preliminary findings strongly suggest that the surrounding extracelllular matrix in cartilage clearly influences the matrix vesicle's ability to mineralize. There is ample precedence for an important interaction between matrix vesicles and their surrounding extracellular environment in growth plate cartilage. While dramatic changes in articular cartilage matrix occur in both osteoarthritis and with calcium crystal deposition, little is known about the interaction of extracellular matrix and matrix vesicles in articular cartilage. We hypothesize that interactions between matrix vesicles and extracellular matrix components strongly influence the ability of articular cartilage matrix vesicles to generate pathologic calcium crystals. In this proposal, we will will use porcine articular cartilage matrix vesicles in a gel-based calcification model to study I) the effects of type II collagen on articular cartilage matrix vesicle mineralization, II) the effects of large and small proteoglycans on articular cartilage matrix vesicle mineralization, and IN) the effects of the calcium binding proteins (osteopontin, SPARC, and matrix gla protein) on articular cartilage matrix vesicle mineralization. The ultimate goal of this work is to understand the pathogenesis of calcium crystal formation in articular cartilage, so that specific therapies for this disabling disease can be designed.